The present invention relates to a process for producing a filter cartridge with bellows and a plastic frame, and to a tool for implementing the process.
Filter cartridges are used, in particular, as gas filters or also as air filters for the passenger space of motor vehicles. They are known in various embodiments and are very frequently provided with a frame, in order to have suitable mechanical strength. For example, a filter cartridge is known from German published patent application 43 23 522. That application discloses a filter cartridge with bellows made of a filter strip material which is folded in a zigzag shape. The bellows is set into a frame that is formed of U-shaped strips, and surrounds the filter bag on all sides, forming a seal. The strips are connected with the filter strip material by bonding. The production of such a filter cartridge is relatively complicated.
Another possibility for the production of a filter insert is shown by German Patent 40 02 078. There, the outside edges of a filter bag are held by comb-like profiles. A material that projects laterally beyond the comb-like profiles is placed around them using contact pressure elements, and the materials are pressed together and fixed in place. However, such a filter does not always have the desired rigidity.
From German Patent 40 39 288, a filter insert is known, where the edge seal is achieved by a closure on the face, with a sealing material that covers it. The sealing material can be a strip that is glued or bonded onto the faces of the bellows. The filter strip material is made of a material that can melt, and its face surfaces are partially melted and compressed in order to achieve an enlarged face surface. For the melting and compression process, the bellows is held in a clamping device. This process is also quite complicated in production. German utility model G 88 08 632 deals with an air filter insert with a frame made of plastic that is cast on, which covers only a small part of the filter body. Here, the flow conditions are unclear, since the air to be filtered is not entirely passed through the filter surface. In addition, the plastic gets into the interstices between folds during the casting process, and takes away some of the filter surface.
Finally, it is also known from German Patent 197 36 267 to form a filter insert that is surrounded on all sides by a frame made of plastic that is injection-molded onto the insert. Here again, there is the problem that plastic can penetrate too far into the interstices between folds.
The invention is based on the object of creating a process for the production of a filter cartridge, and a tool for implementing the process, that are easy to use and where the filter cartridge, with the bellows and the plastic frame, are made in one piece. The cartridge should have as much rigidity as possible, and should be inexpensive to produce.
The object is achieved by the process according to the present invention. A bellows is formed in a known manner from a flat strip of non-woven material, such as paper, or the like. The bellows has the approximate shape of the filter cartridge that it will form. This can be a rectangle or any other geometric shape. The pre-formed bellows is inserted into an injection-molding die and the plastic frame is injection-molded around it, using the injection-molding process. The injection-molding die has a frame shape where the surfaces that lie opposite one another are provided with sawtooth-like profiles. The sawtooth-like profiles engage with one another when the die is closed and hold the individual folds of the bellows in a defined position relative to one another. At edge surfaces of the sawtooth-like profiles, the profiles are structured in such a way that when the die is closed, the bellows is held at its face edges over a narrow region, and pressed together, forming a seal for the injection-molding process. In this connection, the face surfaces of the profiles and the face edges of the bellows are used as an end wall of the frame shape of the injection-molding die.
As the tool for implementing the process, an injection-molding die is used. The injection-molding die has a frame shape that has a bottom die and an upper die. The surfaces of the injection-molding die have sawtooth-like profiles that engage into one another to hold the individual folds of the bellows in a defined position. When the die mold is open, the prefinished bellows are set onto one of the surfaces, with the profiles engaging in the folds. As the die is closed, the profiles of the other cover surface engage in the remaining folds on the other side of the bellows. The profiles are structured in such a way that they form a gap relative to one another, over a small region of the face edges of the bellows, when the die is closed. The gap allows the face edges of the bellows to be securely held, pressed together, and sealed. Using such a tool, the bellows can be provided with a frame all around, using the injection-molding process. This frame securely holds the bellows and results in a definite seal between the bellows and the frame.
In order to be able to hold the bellows securely, it is possible that the profiles have stepped reinforcements at their ends. The surfaces of the stepped reinforcements lie parallel to the surfaces of the rails, and hold the face edge region of the bellows and press it together. The width of the rail reinforcement is selected in such a way that the region of the face edges of the bellows that is covered by the reinforcement is as small as possible.
A particularly advantageous shape of the profiles is achieved if they are provided at their flanks with cleared areas that do not cover their ends. This structure allows a continuous top edge of the profiles, avoiding any steps in the vicinity of the pressing region. The face edges of the bellows are pressed together in the region of the face edges, without any sharp transitions between the interacting ends of the profiles. This embodiment furthermore has the advantage of particularly simple production, since the cleared areas that must be machined into the flanks can be formed from surfaces that stand at a right angle to one another. This geometry of the cleared areas allows the use of standard tools to machine the cleared areas.